The hepatic beta-adrenergic receptor/adenylyl cyclase system is less coupled in early neonatal development than in adult animals; changes in the amounts of G-proteins appear to be responsible for this phenomenon. The overall objective of this project is to assess, in molecular terms, development-dependent changes in subunits of guanine-nucleotide-binding regulatory proteins (G-proteins) in liver and heart during neonatal development and to study the hormonal regulation of G-protein expression in the neonate. The proposed studies will assess whether changes in the amounts of G-protein subunits are accompanied by differential gene expression. mRNA levels for G-proteins as well as transcriptional rates will be measured. Because adrenergic agonists play an important role in cardiac function and disease, major efforts will be made in cardiac tissue and to develop a myocyte model for studying G-protein gene expression as a function of neonatal development and hormonal regulation. The specific aims are: (1) to assess developmental expression of G-protein subunits in neonatal liver and heart by quantifying them at both protein and mRNA levels, using specific antibodies and molecular hybridization probes; (2) to measure the effect of glucocorticoid (dexamethasone) on the developmental expression of G-protein subunits; (3) to measure transcriptional rates of G-protein subunits by nuclear run-off assay as a function of perinatal development and in response to glucocorticoid treatment; and (4) to study hormonal regulation of developmental expression of G-proteins using primary cultures of cardiac myocytes developed from neonatal rabbits as a means to infer the molecular events underlying G-protein expression. These studies should be valuable in understanding molecular mechanisms underlying hormonal responsiveness in liver and heart during neonatal development.